Generally, a bent axis type hydraulic machine has the drive disc of a rotational shaft coupled with a cylinder block through pistons which are reciprocally received in the cylinder block. Therefore, when the bent axis type hydraulic machine is used as a hydraulic pump, the hydraulic reaction forces acting on pistons on the high pressure side in the discharge stroke are supported by the rotational shaft through the drive disc. Similarly, when applied as a hydraulic motor, the hydraulic reaction forces acting on pistons on the high pressure side in the suction (feeding) stroke are supported by the rotational shaft through the drive disc.
Accordingly, in a bent axis type hydraulic machine of this type, the rotational shaft is subject to radial and thrust load of the hydraulic reaction forces and therefore it is necessary to hold the rotational shaft in suitable condition for supporting these loads.
In this regard, it has been the general practice in the prior art to resort to the so-called mechanical support type which mechanically supports the rotational shaft rotatably by ball or roller bearings capable of supporting the radial and thrust loads, with the partial hydrostatic support type which mechanically supports either the radial or thrust loads by a roller or ball bearing while supporting the other load hydraulically by a hydrostatic bearing, or the total hydrostatic support type which supports the entire loads hydraulically by hydrostatic bearings.
Of these various shaft supporting means, a hydraulic machine employing a shaft support bearing of the partial hydrostatic type is described in, for example, Japanese Laid-Open Patent Application 60-224981, wherein a rotational shaft is supported by a hydrostatic thrust bearing composed of a stationary bearing and a movable bearing, each movable bearing being provided with springs and an outer ring to counteract the thrust load which acts on the rotational shaft, along with pistons which are located on the side of the outer ring to generate a pressure in the same direction as the springs and to which oil pressure is applied from the high pressure area in the cylinder block.
On the other hand, a hydraulic machine supporting a shaft by total-hydrostatic bearings is described in Japanese Laid-Open Patent Application 59-131776, which is provided with a radial load bearing sleeve and a thrust load bearing plate within a casing, in combination with a drive flange which is movably disposed between the bearing sleeve and the bearing plate to serve also as a drive disc. The one end face of the drive flange is securely connected to a rotational shaft and the other end face is coupled with pistons. Further, pressure chambers, constituting a hydrostatic radial bearing, are defined between the outer peripheral surface of the drive flange and the bearing sleeve, and drive shoes, constituting a hydrostatic thrust bearing, are provided on one end face of the drive flange. The pistons have oil passages bored therein for supplying high pressure oil to the radial and thrust bearings from cylinders in the cylinder block, to thereby hydrostatically support the radial and thrust loads.
In this connection, even if the hydraulic reaction forces which are applied to the rotational shaft through pistons are the same, the resulting radial and thrust loads vary in dependence upon the tilt angle of the cylinder block. More specifically, the radial load F.sub.R and thrust load F.sub.T are expressed as ##EQU1## where: F is the hydraulic reaction force by the piston,
.THETA. is the angle of inclination or tilt angle, PA1 F.sub.R is the radial load, and PA1 F.sub.T is the thrust load.
When the tilt angle .THETA. is minimum, the radial load F.sub.R becomes minimum while the thrust load F.sub.T becomes maximum. On the other hand, when the tilt angle .THETA. is maximum, the radial load F.sub.R becomes maximum while the thrust load F.sub.T becomes minimum.
In short, the above-described prior art devices are arranged to directly supply the hydrostatic bearing or bearings with high pressure oil of a certain level which is generated in cylinders on the high pressure side of a cylinder block (a pump) or which is fed to cylinders on the high pressure side (a motor).
In this manner, in spite of the fact that the radial and thrust loads of the hydraulic reaction forces vary in synchronism with variations in tilt angle of the cylinder block, the conventional counterparts have been arranged simply to apply a high oil pressure of a certain level to a hydrostatic bearing. It follows that the hydrostatic bearing has constant characteristics in load supporting capacity, more specifically, in statically hydrodynamic and dynamically hydrodynamic loads supporting capacity, forming an oil film of an increased thickness on the guide surface of the hydrostatic bearing when the load of the hydraulic reaction force is of a light one (i.e., when the hydrostatic bearing capacity is higher than the load of the hydraulic reaction force), balancing the hydrostatic bearing capacity with the load of a hydraulic reaction force in supporting the latter.
The support of this sort has a problem that the thickness of the oil film is increased to an excessive degree. In this connection, it is known that the rate of oil leakage from an oil film formed on a given sliding surface is proportional to the cube of the oil film thickness. An oil film which has an excessively large thickness as mentioned hereinbefore involves a greater rate of oil leakage from the hydrostatic bearing guide surface, which will lead to a problem of increased power loss.
On the other hand, in a case where the tilt angle of the cylinder block is frequently changed during operation of a pump or motor, the support capacities of the hydrostatic thrust and radial bearings are varied each time when the cylinder block is tilted. This will be reflected by degradations in accuracy of the drive disc positioning in the radial and thrust directions, increasing vibrations of the hydraulic machine to such a degree as will hinder stable rotational movements in high speed operation and impair the durability of the machine.
The present invention solves the above-mentioned problems or drawbacks of the prior art, and has as its object the provision of a bent axis type variable displacement hydraulic machine employing a partial and/or total-hydrostatic bearing support which can ensure operations with reduced oil leakage and of high stability and reliability even under conditions involving intermittent or continual changes of the tilt angle of the cylinder block